Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes.
When a fluid working machine operates as a pump, a low pressure manifold typically acts as a net source of fluid and a high pressure manifold typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold typically acts as a net source of fluid and a low pressure manifold typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” are relative, with the relative pressures being determined by the application. In some embodiments of the present invention the pressure within the low pressure manifold is significantly higher than atmospheric pressure, for example, several atmospheres, however, it will be less than the pressure in the high pressure manifold during normal operation. A fluid working machine may have more than one low pressure manifold and more than one high pressure manifold.
Large displacement ring cam fluid working machines (i.e. those having a large rotating annular cam driving a multitude of pistons arranged around the cam, with each piston cycling multiple times per cam revolution) are known and are proposed for use in renewable energy applications in which there is a low speed rotating input but a relatively high speed electrical generator (Rampen, Taylor & Riddoch, Gearless transmissions for wind turbines, DEWEK, Bremen, December 2006). Ring cam fluid working machines typically have a plurality of rollers rolling on a wave shaped cam and driving pistons within cylinders. Either the pistons and cylinders may rotate inside the cam or the cam may rotate inside the pistons and cylinders. Such pumps driving relatively small hydraulic motors have been proposed as robust variable speed transmissions not just for wind turbine generators, but also for tidal stream and gravity-fed hydroelectric generators. Variable displacement could be added to the pumps or the motors, or both, using the operating principle above, for example.
However, large ring cam machines are difficult and expensive to repair, requiring disassembly of the whole body to repair even one working chamber. This is particularly expensive in renewable energy applications because the heavy pump must be brought to the surface, requiring a large and costly crane at typically a remote location.
Accordingly, the invention aims to provide a fluid working machine which is readily repairable in situ, even if the machine is massive (e.g. greater than 500 kg) and in a location which is difficult to access.
EP 0 692 071 proposed a modular construction in which the ring cam is made up of segments having confronting ends joined at a tongue and groove formation across which the rollers roll, and bolted via flanges to a rotating shaft. It was also proposed to provide removable piston and cylinder carrying units, bolted to each other around or within the ring cam. This machine suffers from difficulty of repair in situ in a working application because the supporting structure of the machine is broken when the pumping modules are bolted to a surrounding frame which prevents their removal without dismantling the frame.
EP 1 985 853 (Golle) discloses a pump from which piston cylinders can be demounted, but only partially exposing an internal ring cam surface. This makes it impossible to maintain the ring cam without removal of the shaft from the pump. Also, the pump of Golle includes a single metal body with bores within which the piston cylinders are received and, accordingly, it is relatively heavy for a given fluid displacement and it would be difficult to access and maintain components of the pump if it were built to a large scale suitable for, e.g. a wind turbine generator nacelle.
Accordingly, the invention aims to provide a fluid working machine which can retain its structural integrity during maintenance. Related to this, large fluid working machines for high torque applications require a structure which retains its structural integrity despite that substantial forces arising from the weight of the machine and the torque of the load (in the case of a motor) or input (in the case of a pump). This can be most simply obtain by providing large structural elements, for example, a large chassis or housing. However, the mass of the resulting components can be substantial and the invention aims to provide a structure which has sufficient mechanical strength while minimising the mass of the structural elements which are required to maintain integrity in use.